1. Field of the Invention
The invention relates to a battery pack of a secondary battery such as a lithium ion battery or the like.
2. Description of the Related Art
Since a lithium ion battery is weak against an overcharge or an overdischarge, generally, it has a construction of a battery pack in which a battery cell and a protecting circuit are integrated. The protecting circuit has three functions of protection against the overcharge, the overdischarge, and an overcurrent. Those protecting functions will be simply explained hereinbelow.
The overcharge protecting function will now be described. When the lithium ion battery is charged, a battery voltage continues to rise even after exceeding a full charge state. Such an overcharge state can cause a danger. Therefore, it is necessary to charge the battery at a constant current and a constant voltage and at a charge control voltage that is equal to or less than a rated value (for example, 4.2 V) of the battery. However, there is a risk of occurrence of the overcharge due to a failure of a charger or use of a charger for a different kind of battery. When the battery is overcharged and the battery voltage is equal to or more than a certain value, the protecting circuit turns off a charge control FET (Field Effect Transistor), thereby shutting off a charge current. Such a function is the overcharge protecting function.
The overcharge protecting function will be further described with reference to FIGS. 14 to 16. FIG. 14 is a schematic diagram showing a normal state of a lithium ion battery pack 150. The lithium ion battery pack 150 has two cells 152 and 153, a discharge control FET 154, and a charge control FET 155. An IC for control 151 measures voltages at a plurality of predetermined positions, controls the discharge control FET 154 and the charge control FET 155 in accordance with results of the measurement, and shuts off a flow of a current in a predetermined direction. The discharge control FET 154 is controlled by a signal which is sent from the control IC 151 through a signal line 156. The charge control FET 155 is controlled by a signal which is sent from the control IC 151 through a signal line 157.
The lithium ion battery pack 150 is connected to a load 158 and a charger 159. Arrows (A and B) indicative of the directions of the currents flowing in the discharge control FET 154 and the charge control FET 155 are shown in a lower portion of FIG. 14. That is, when the load 158 is connected, a discharge current flows in the direction shown by the arrow A and when the charger 159 is connected, a charge current flows in the direction shown by the arrow B.
FIG. 15 is a schematic diagram showing a state of a lithium ion battery pack 160 in the case where the foregoing overcharge protecting function acts. An IC for control 161 detects that, for example, a voltage of either a cell 162 or a cell 163 is equal to or more than 4.3 V±50 mV and controls so as to turn off a charge control FET 165 (a discharge control FET 164 is ON), thereby shutting off the charge current. In this case, however, since the discharge current by a load 168 flows through a parasitic diode, only a charge by a charger 169 is inhibited.
The overdischarge protecting function will now be described. When the battery is discharged down to a rated discharge terminating voltage or less and enters the overdischarge state where the battery voltage is equal to or less than, for example, 2 to 1.5 V, there is a case where the battery fails. When the battery is discharged and the battery voltage is equal to or less than a certain voltage value, the protecting circuit turns off a discharge control FET, thereby shutting off a discharge current. Such a function is the overdischarge protecting function.
FIG. 16 is a schematic diagram showing a state of a lithium ion battery pack 170 in the case where the foregoing overdischarge protecting function acts. An IC for control 171 detects that, for example, a voltage of either a cell 172 or a cell 173 is equal to or less than 3.0 V and controls so as to turn off a discharge control FET 174 (a charge control FET 175 is ON), thereby shutting off the discharge current. In this case, however, since the charge current by a charger 179 flows through a parasitic diode, only a discharge by a load 178 is inhibited.
The overcurrent protecting function will now be described. When (+) and (−) terminals of the battery are short-circuited, a large current flows, so that there is a risk of abnormal heat generation. When a discharge current of a certain value or more flows, the protecting circuit turns off the discharge control FET, thereby shutting off the discharge current. Such a function is the overcurrent protecting function.
Although an explanation in conjunction with diagrams is omitted with respect to the overcurrent protecting function, the overcurrent protecting function is fundamentally similar to the overdischarge protecting function and is controlled so that the charge control FET is turned on when the discharge control FET is OFF.
As mentioned above, in the battery pack of the secondary battery such as a lithium ion battery or the like, the protecting functions are realized by controlling the discharge control FET and the charge control FET. The control of those FETs is made by applying a battery voltage to a gate terminal of each FET. A resistance (ON resistance) between a drain and a source of the FET depends on the voltage which is applied to the gate terminal.
Such a relation is shown in a graph of FIG. 17. For example, when the gate voltage is equal to 3 V, the resistance between the drain and the source is equal to 18.1 mΩ and when the gate voltage is equal to 10 V, the resistance between the drain and the source is equal to 12.8 mΩ. In the case of the battery pack comprising one lithium ion battery, the battery voltage usually changes from 4.2 V to 3.0 V. Therefore, the resistance of the FET changes from 15.8 mΩ to 18.1 mΩ.
Hitherto, a battery pack which comprises a protecting circuit including a discharge switch, a charge switch, and the like and outputs a predetermined voltage by combining a secondary battery and a step-down type voltage converter has been disclosed in JP-A-7-7864.
However, there was not such an idea that the resistance of the FET upon discharging and that upon charging are reduced by applying a signal of a high voltage to the gate terminals of the discharge control FET and the charge control FET. Since there are such problems that when the resistance of the FET is large, a voltage drop due to the FET at the time of the large current discharge increases and the like, it is unpreferable. For example, the voltage drop at the time of the discharge of 2 A is equal to 36.2 mV when the gate voltage is equal to 3 V, and it is equal to 31.6 mV when the gate voltage is equal to 4.2 V. If the gate voltage is high, a voltage loss and an energy loss decrease, so that a duration time of a main body using such a battery pack can be prolonged.